This invention relates to an apparatus and method for signaling the detection of mains power loss by locally powered equipment which has a communications link established with central equipment having backup battery power. Such detection of loss of mains power is often used to trigger housekeeping operations in a processor, whereby the processor shuts down in an orderly manner after the loss of mains power. This housekeeping may include the transmission of a power fail message to remote equipment prior to the loss of the mains-derived voltage which operates the processor.
Communications equipment is often sensitive to the loss of mains power. In some systems, either a standby source of mains power is provided, or a method of detecting the loss of mains power is employed in conjunction with an energy storage element which provides enough energy for the powered equipment to complete any tasks that must be finished before the energy storage element is depleted.
One class of system extensively discussed in the prior art is one which senses a power fail event, and communicates this to a processor or system. U.S. Pat. No. 4,509,201 by Sekigawa et al discloses a voltage monitor coupled to a battery which signals when the battery has reduced operating level, thereby producing a signal to a CPU. U.S. Pat. No. 4,509,874 by Shimamoto et al. includes a variable A/D converter which measures the output voltage of two batteries and an AC/DC converter for use by a CPU. U.S. Pat. No. 5,831,805 by Sekine discloses a power failure detection subsystem for determining the loss of power and sending this information to a CPU.
A second class of system uses this loss of power signal to start a process related to saving information from a pre-existing process. U.S. Pat. No. 5,283,792 by Davies et al, U.S. Pat. No. 5,339,446 by Yamasaki et al, and U.S. Pat. No. 5,423,045 by Kannan et al. disclose the storage of CPU data and instructions upon the detection of power failure or imminent loss of power.
A third class of system uses a local capacitor to store sufficient charge to enable a system to operate on the content of this charge alone. U.S. Pat. No. 5,553,138 by Heald et al discloses a CPU which derives its power from a telephone hook voltage, and senses when the level of voltage on this storage capacitor is diminished, thereby producing a signal.
A power supply furnishes a plurality of voltages used as the power sources for various parts of a communication system. A CPU and a primary physical layer interface are powered using a first output voltage which persists for a holdup time after the loss of mains power. The remaining power supply secondary voltages are used for secondary physical layer interfaces, and other functions not related to the operation of either the primary physical interface, or the operation of the CPU. These secondary voltages have two values: an operating voltage related to the operation of the associated loads, and a signaling voltage for the communication of the detected AC mains power failure. When the power supply detects the loss of mains voltage, one or more of these power supply output secondary voltages is quickly pulled to this signaling voltage after the loss of mains power. A set of comparators examines one or more of the secondary power supply voltages, and these comparators generate an interrupt to the CPU if one or more of the other voltages is near the signaling voltage. In response to this detected signaling voltage, the CPU sends a special xe2x80x9cpower failxe2x80x9d message across the primary communications link to indicate to the remote device on this link that the provision of mains power has been terminated. In this manner, the remote device can distinguish between the loss of power to the remote device, and the loss of link through a bad interface, or a bad physical link.
A first object of the invention is the detection and communication of a mains power failure event in equipment that is powered by a plurality of power supply voltages. A second object of the invention is the use of one or more power supply first output voltages for powering equipment required for sending messages on a primary communications link, and the use of one or more power supply secondary output voltages for both the operation of secondary loads, as well as the signaling of loss of power information. A third object of the invention is the provision of power to a power fail messaging circuit for a holdup time that permits the sending of a power fail message after the detection of loss of power. A fourth object of the invention is the use of series signalers and comparators to send and receive power fail signals. A fifth object of the invention is the use shunt signalers and comparators to send and receive power fail signals.